@@ -175,6 +175,39 @@ $seam->locks->unlock_door(
 );
 ```
 
+### Setting a param to null
+
+The Seam API distinguishes three states for an updatable param:
+omitted (leave the stored value unchanged), null (unset the stored value),
+and a value (set it).
+
+PHP's `null` means omitted.
+The SDK removes `null` params from the request entirely,
+so passing `null` never unsets a value.
+To unset a value, pass the `Seam\NullValue::NULL` sentinel,
+which the SDK sends as JSON `null` in request bodies
+and as an empty value in query strings:
+
+```php
+use Seam\NullValue;
+
+// Leaves the name unchanged.
+$seam->devices->update(device_id: $device_id, name: null);
+
+// Unsets the name.
+$seam->devices->update(device_id: $device_id, name: NullValue::NULL);
+```
+
+The other Seam SDKs spell the sentinel `NULL` and its type `Null`,
+but those names are reserved in PHP, so both live on one enum:
+`Seam\NullValue` is the type, and its single case `NullValue::NULL`
+is the value to pass.
+
+Only pass `NullValue::NULL` for params the API documents as nullable.
+Generated methods type nullable params as a union with the sentinel,
+e.g. `string|NullValue|null`, so passing it anywhere else fails with a
+`TypeError`.
+
 ### Pagination
 
 Some Seam API endpoints that return lists of resources support pagination.
@@ -388,6 +421,69 @@ $client = new GuzzleHttp\Client([
 $seam = Seam\Seam::from_client($client);
 ```
 
+#### Serializing URL search params
+
+The Seam API parses URL search params as complex types.
+If you call it with your own HTTP client,
+`Seam\StrictUrlSearchParamsSerializer` is exported for that purpose.
+The `_strict=true` param is added to any non-empty query
+so the Seam API uses strict, schema-aware parsing.
+A query with no serializable params remains empty.
+
+```php
+use Seam\StrictUrlSearchParamsSerializer;
+
+$query = StrictUrlSearchParamsSerializer::serialize([
+    "device_ids" => ["device1", "device2"],
+]);
+
+$response = file_get_contents(
+    "https://connect.getseam.com/devices/list?{$query}",
+    context: stream_context_create([
+        "http" => ["header" => "Authorization: Bearer your-api-key"],
+    ]),
+);
+```
+
+The serialization defines the name and value of each search param,
+where every value is a string.
+`Seam\UrlSearchParams` holds those pairs and renders the query string,
+as [URLSearchParams] does for the [reference implementation]:
+
+```php
+use Seam\StrictUrlSearchParamsSerializer;
+use Seam\UrlSearchParams;
+
+$search_params = new UrlSearchParams();
+
+StrictUrlSearchParamsSerializer::update($search_params, [
+    "device_ids" => ["device1", "device2"],
+]);
+
+iterator_to_array($search_params);
+// => [["device_ids", "device1"], ["device_ids", "device2"], ["_strict", "true"]]
+
+(string) $search_params;
+// => 'device_ids=device1&device_ids=device2&_strict=true'
+```
+
+Pass either the query string or the pairs to your HTTP client.
+A client may percent-encode a few characters differently than
+`URLSearchParams` does, e.g. Guzzle escapes `*` and leaves `~` unescaped,
+which the Seam API reads as the same params either way.
+
+A param set to `null` is omitted,
+while a param set to `NullValue::NULL` is serialized to an empty value,
+which the Seam API reads as null,
+as described in [Setting a param to null](#setting-a-param-to-null).
+A param that cannot be represented raises a `Seam\UnserializableParamError`.
+
+The Seam API parses these params with the corresponding [parser].
+
+[URLSearchParams]: https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams
+[reference implementation]: https://github.com/seamapi/url-search-params-serializer
+[parser]: https://github.com/seamapi/url-search-params-parser
+
 #### Errors
 
 Every exception the SDK raises implements `Seam\SeamException`, so it can be
 
@@ -9,6 +9,7 @@ use {{this}};
 use GuzzleHttp\Client;
 use GuzzleHttp\ClientInterface;
 use Seam\Http\ClientFactory;
+use Seam\Http\SerializingClient;
 
 /**
  * Client for the Seam API.
@@ -33,6 +34,10 @@ class Seam
 
   /**
    * The Guzzle client this instance makes its requests with.
+   *
+   * Query params given as a map and NullValue::NULL sentinels in JSON
+   * bodies are serialized with the Seam standard before the request goes
+   * out; see Seam\Http\SerializingClient.
    */
   public ClientInterface $client;
 
@@ -78,13 +83,13 @@ class Seam
       "timeout" => $timeout,
     ]);
 
-    $this->client = $client ?? ClientFactory::create(
+    $this->client = SerializingClient::wrap($client ?? ClientFactory::create(
       Options::get_endpoint($endpoint),
       Auth::get_auth_headers($api_key, $personal_access_token, $workspace_id),
       $guzzle_options,
       $retries,
       $timeout
-    );
+    ));
 
 {{#each parentClients}}
     $this->{{namespace}} = new {{clientName}}Client($this->client, $this->defaults);
 
@@ -10,6 +10,7 @@ import {
 
 const clientInterfaceClass = 'GuzzleHttp\\ClientInterface'
 const bodyClass = 'Seam\\Http\\Body'
+const nullValueClass = 'Seam\\NullValue'
 const resolveActionAttemptClass = 'Seam\\Http\\ResolveActionAttempt'
 const resourcesNamespace = 'Seam\\Resources'
 
@@ -74,6 +75,21 @@ const onResponseParameter = {
   required: false,
 }
 
+// A nullable param accepts the NullValue::NULL sentinel, which sends an
+// explicit null to unset a value. A merely optional param does not: optional
+// means omit by passing null, and sending null there would unset a value
+// instead. Optionality composes with nullability rather than replacing it.
+const getParameterPhpType = (parameter: {
+  type: string
+  isOptional: boolean
+  isNullable: boolean
+}): string => {
+  const { type, isOptional, isNullable } = parameter
+  if (type === 'mixed') return type
+  if (isNullable) return `${type}|NullValue${isOptional ? '|null' : ''}`
+  return `${isOptional ? '?' : ''}${type}`
+}
+
 const getMethodLayoutContext = (
   method: PhpClientMethod,
 ): MethodLayoutContext => {
@@ -96,7 +112,7 @@ const getMethodLayoutContext = (
   const signatureParams = sortedParameters
     .map(
       (p) =>
-        `${(p.isNullable || p.isOptional) && p.type !== 'mixed' ? '?' : ''}${p.type} $${p.name}${p.isOptional ? ' = null' : ''}`,
+        `${getParameterPhpType(p)} $${p.name}${p.isOptional ? ' = null' : ''}`,
     )
     .concat(
       usesActionAttempt
@@ -109,7 +125,7 @@ const getMethodLayoutContext = (
   const documentedEndpointParameters = sortedParameters.map(
     ({ name, type, description, isOptional, isNullable }) => ({
       name,
-      type,
+      type: isNullable && type !== 'mixed' ? `${type}|NullValue` : type,
       description,
       required: !isOptional,
       isOptional,
@@ -165,9 +181,16 @@ const getUseStatements = (client: PhpClient): string[] => {
   // Void endpoints never read the response, so they do not decode it.
   const readsBody = client.methods.some((m) => m.returnResource !== '')
 
+  // Only nullable params reference the null sentinel type; importing it
+  // elsewhere would trip the unused-import lint.
+  const usesNullValue = client.methods.some((m) =>
+    m.parameters.some((p) => p.isNullable && p.type !== 'mixed'),
+  )
+
   return [
     clientInterfaceClass,
     ...(readsBody ? [bodyClass] : []),
+    ...(usesNullValue ? [nullValueClass] : []),
     ...(usesActionAttempt ? [resolveActionAttemptClass] : []),
     ...[...resourceNames].map((name) => `${resourcesNamespace}\\${name}`),
   ].sort((a, b) => a.localeCompare(b))
 
@@ -0,0 +1,117 @@
+<?php
+
+namespace Seam\Http;
+
+use GuzzleHttp\ClientInterface;
+use GuzzleHttp\Promise\PromiseInterface;
+use Psr\Http\Message\RequestInterface;
+use Psr\Http\Message\ResponseInterface;
+use Seam\NullValue;
+use Seam\StrictUrlSearchParamsSerializer;
+
+/**
+ * Applies the Seam serialization standard to every request the wrapped
+ * client sends.
+ *
+ * Query params given as a map are serialized with
+ * StrictUrlSearchParamsSerializer and handed to Guzzle as a raw query
+ * string, because Guzzle's own encoder follows different rules: it escapes
+ * `*`, leaves `~` alone, and drops an empty array instead of sending
+ * `name=`. A query already given as a string is a representation the
+ * caller chose, so it passes through untouched.
+ *
+ * NullValue::NULL sentinels in a JSON body become JSON null, so the same
+ * sentinel works whether a route sends a query string or a body.
+ */
+final class SerializingClient implements ClientInterface
+{
+    private function __construct(private ClientInterface $client) {}
+
+    /**
+     * Wraps a client, or returns it unchanged when it is already wrapped.
+     */
+    public static function wrap(ClientInterface $client): self
+    {
+        return $client instanceof self ? $client : new self($client);
+    }
+
+    #[\Override]
+    public function send(
+        RequestInterface $request,
+        array $options = [],
+    ): ResponseInterface {
+        return $this->client->send($request, self::serialize_options($options));
+    }
+
+    #[\Override]
+    public function sendAsync(
+        RequestInterface $request,
+        array $options = [],
+    ): PromiseInterface {
+        return $this->client->sendAsync(
+            $request,
+            self::serialize_options($options),
+        );
+    }
+
+    #[\Override]
+    public function request(
+        string $method,
+        $uri = "",
+        array $options = [],
+    ): ResponseInterface {
+        return $this->client->request(
+            $method,
+            $uri,
+            self::serialize_options($options),
+        );
+    }
+
+    #[\Override]
+    public function requestAsync(
+        string $method,
+        $uri = "",
+        array $options = [],
+    ): PromiseInterface {
+        return $this->client->requestAsync(
+            $method,
+            $uri,
+            self::serialize_options($options),
+        );
+    }
+
+    #[\Override]
+    public function getConfig(?string $option = null)
+    {
+        return $this->client->getConfig($option);
+    }
+
+    /**
+     * @param array<string, mixed> $options
+     * @return array<string, mixed>
+     */
+    private static function serialize_options(array $options): array
+    {
+        if (
+            isset($options["query"]) &&
+            ($options["query"] instanceof \stdClass ||
+                is_array($options["query"]))
+        ) {
+            $serialized = StrictUrlSearchParamsSerializer::serialize(
+                $options["query"],
+            );
+
+            if ($serialized === "") {
+                unset($options["query"]);
+            } else {
+                $options["query"] = $serialized;
+            }
+        }
+
+        if (array_key_exists("json", $options)) {
+            $options["json"] = NullValue::replace($options["json"]);
+        }
+
+        return $options;
+    }
+}
@@ -0,0 +1,67 @@
+<?php
+
+namespace Seam;
+
+/**
+ * The explicit null sentinel used by request params.
+ *
+ * PHP has a single absence value, null, but the Seam API distinguishes an
+ * omitted param from a param explicitly set to null. For example, in an
+ * update request, an omitted param leaves the current value unchanged,
+ * while a null param unsets the current value.
+ *
+ * Since sending null is rarely intended and unsetting a value cannot be
+ * undone, null means the safe option of omitting the param. Sending null is
+ * explicit and always spelled NullValue::NULL:
+ *
+ * ```php
+ * use Seam\NullValue;
+ * use Seam\UrlSearchParamsSerializer;
+ *
+ * UrlSearchParamsSerializer::serialize(["name" => NullValue::NULL, "limit" => 20]);
+ * // => 'limit=20&name='
+ *
+ * UrlSearchParamsSerializer::serialize(["name" => null, "limit" => 20]);
+ * // => 'limit=20'
+ * ```
+ *
+ * Use it wherever the Seam API documents null as a meaningful value, e.g.,
+ * to unset a value in an update request, or to filter by an unset value.
+ */
+enum NullValue
+{
+    /**
+     * Sentinel for a param explicitly set to null.
+     */
+    case NULL;
+
+    /**
+     * Returns a copy of a value with every NullValue::NULL replaced by null.
+     *
+     * The sentinel only distinguishes an explicit null from an omitted param
+     * within this SDK. Once a request body is being serialized, the param is
+     * known to be present, so the sentinel becomes the null that JSON has.
+     *
+     * Recurses into arrays and stdClass objects without mutating them; every
+     * other value is returned unchanged.
+     */
+    public static function replace(mixed $value): mixed
+    {
+        if ($value instanceof self) {
+            return null;
+        }
+
+        if (is_array($value)) {
+            return array_map(self::replace(...), $value);
+        }
+
+        if ($value instanceof \stdClass) {
+            return (object) array_map(
+                self::replace(...),
+                get_object_vars($value),
+            );
+        }
+
+        return $value;
+    }
+}